A significant stock decline in Japan erased $15 billion from SoftBank Group Corp.'s value on Monday, marking the company's steepest one-day fall since 1998. The technology conglomerate's shares plummeted 19%, extending their September quarter drop to 38%, the worst since 2001. This downturn coincides with founder Masayoshi Son's planned investment surge in AI and semiconductor technologies, amid a broader market rout that threatens to impact SoftBank’s Vision Fund, which holds stakes in numerous tech startups.

The stock collapse also wiped out $2.9 billion of Son’s personal wealth in one day, totaling a $5 billion loss over the past three trading days. This setback has erased much of Son's gains since the year's start, making him one of Asia's hardest-hit billionaires. According to Bloomberg Intelligence analysts, Marvin Lo and Chris Muckensturm, the decline reflects diminishing enthusiasm for AI as investors scrutinize these companies' ability to generate revenue and profit. They noted that while SoftBank’s AI investment strategy might eventually restore profitability, it faces high execution risks.